Two of the dead were Israelis, according to Israeli Foreign Ministry spokesman Alon Lavi. The United States said two of its citizens were killed, U.S. National Security Council spokesman Ned Price said.

A source within the Israeli government confirmed to CNN that the two Israelis killed were dual American-Israeli citizens.

Iran state media news agency IRNA reported that an Iranian was killed.

Thirty-six people were injured in the blast, Turkish Health Minister Mehmet Muezzinoglu told reporters. Seven were in critical condition and four were undergoing surgery.

"Those who kill are killing humanity," Muezzinoglu said. "I condemn those who are killing humanity like this."

Among the injured were 11 other Israelis, Lavi said.

Israeli Prime Minister Benjamin Netanyahu said it was unclear whether the suicide bombing was targeting Israelis.

Israel is sending two planes to Turkey to bring the injured and deceased home, Netanyahu said in a televised statement.

Preliminary reports indicated that 12 of the injured are from other countries, according to Muezzinoglu.

Price said the United States is steadfast in its support for Turkey.

"These repeated acts of terrorism in Turkey must come to an end," Price said in a written statement. "We are in close touch with Turkish authorities and reaffirm our commitment to work together with Turkey to confront the evil of terrorism."

No claim of responsibility

Israel's Ministry of Foreign Affairs was holding an emergency meeting to assess the situation. Since August 2014, Israel has advised its citizens to avoid nonessential travel to Turkey.

A number of Irish citizens were injured, according to Ireland's foreign minister, Charlie Flanagan.

Police cordoned off Taksim Square after the attack, as helicopters flew overhead and multiple ambulances gathered at the site.

Stunned shoppers ran away from the scene, some in tears.

No group immediately claimed responsibility.

In a statement, NATO Secretary General Jens Stoltenberg condemned the attack as "yet another terrorist outrage targeting innocent civilians and our Ally Turkey."

"There can be no justification for terrorism," the statement said. "NATO Allies stand with Turkey, united in our determination to fight terrorism in all its forms."

Days after deadly car bomb in Ankara

The attack comes nearly a week after a car bomb ripped through a busy square in the capital of Ankara on March 13.

The Kurdistan Freedom Falcons -- or TAK, a militant offshoot of the Kurdish separatist group, PKK -- boasted that it was behind that bombing, which killed 37 people. The group said on its website Thursday that its militants struck "in the heart of [the] fascist Turkish republic."

The PKK, or Kurdistan Worker's Party, seeks an independent state in Turkey, and has been in an armed struggle with the government for decades. The United States and EU have designated both it and the TAK as terrorist organizations.

A ceasefire between the PKK and Turkey fell apart last summer, after which Turkey bombed the terror group's positions in northern Iraq while also imposing curfews in crackdowns on heavily Kurdish areas on southeastern Turkey.

Some residents have accused Turkey of unjust collective punishment, saying security forces have acted with impunity and killed civilians.

Kurdish militant group warned tourists

The Kurdistan Freedom Falcons specifically have been tied to a number of horrific attacks on their own.

This includes a February bombing targeting military vehicles in central Ankara that killed 28 people.

The TAK called that attack "revenge" for Turkish military actions and threatened more violence -- warning foreigners, especially, to stay away from Turkey.

"Tourism is one of the important sources feeding the dirty and special war, so it is a major target we aim to destroy," the TAK said then in an English-language statement. "We warn the foreign and native tourists not to go to the tourist ... areas in Turkey.

"We are not responsible for who will die in the attacks targeting those areas," the militant group said, adding that Turkey is unable "to save you and its own people."